In processing filled or reinforced plastics or plastics mixtures, problems frequently arise in connection with mixing the individual components of the mixture that will be injection molded. Particularly in the case of long-fiber reinforcement materials (e.g., glass fibers, carbon fibers, natural fibers such as wood), it is not only necessary to treat the material gently so as to damage the fibers as little as possible, but also to distribute the material as uniformly as possible. Problems arise especially when fillers such as e.g., talc, chalk, mineral fillers, or glass fibers are to be admixed to a basic plastic mass, and the mixture is to be homogenized.
Mixing chambers are indeed known, e.g., from the DE 74 14 436 U1. Multi-component plastics, especially polyurethane, are processed thereby conducting the individual components of the mixture to a mixing chamber. The individual components are transported to the mixing chamber through feedlines. At the same time, an agitator, situated in the mixing chamber, is started. The mixed plastic thus also begins to exit from the outlet opening of the mixing chamber into the mold cavity.
Initial problems arise when the plastic base material of the mixture is to be processed in a classical injection molding machine, which generally is equipped with a reciprocating screw plasticizing and injection unit. Difficulties that arise because the discontinuous plasticizing process acts on the mixing process in the mixing element, which operates best continuously.
The prior art contains hardly any examples of a solution if plastics with fibrous or powdered fillers or reinforcing materials are to be mixed and homogenized.
The only solutions known for this are such where the plasticizing and injection screw effects both the melting and the homogenizing process; the plastic base material as well as the fillers are conducted into the mixing region of the screw. But such solutions do not lead to an especially good result: Melting, metering, uniform mixing, and injecting are component processes which require different process conditions. In such a unit, the process becomes possible only by accepting compromises. Especially with long-fiber additives or fillers, the fibers can be substantially damaged, which impairs the quality of the finished molded part.
Consequently, solutions have also been used, which have tried to meter the fillers into a plastic base mass through specially designed screws. In this connection, double-screw extruders should especially be mentioned, but these require continuous operation, while the injecting process must proceed discontinuously. This disadvantageously entails the necessity of quite complicated coordination, so that the continuously operating double-screw extruder, with a prescribed output, can be matched to the intermittently operating injection molding process.
As a result, the invention is directed to a method and an associated apparatus by means of which it becomes possible to admix arbitrary amounts of additives in the form of fibrous or powdered fillers or reinforcement materials, especially glass fibers, carbon fibers, wood fibers, chalk, mineral fillers, and/or talc to the plastics which have been melted in classical reciprocating screw plasticizing and injection units, and to homogenize the resulting mixture. That is, various components are to be compounded directly in the injection molding machine, and the components are to be mixed with one another. An essential feature here is that the homogeneous mixture is achievable when long-fiber additives are admixed, without any, or only limited, destruction of the fibers.
In general, in accordance with an aspect of the present invention, a process includes the following steps: a) plasticizing a mixture of the plastic material with the fillers or reinforcement materials contained therein, in a reciprocating screw plasticizing and injection unit, by rotating a plasticizing and injection screw, which is disposed in a preplasticizing cylinder so that it can move rotationally and axially; b) expelling the plasticized mixture of plastic material and filler or reinforcement material by the axial movement of the plasticizing and injection screw, from the reciprocating screw plasticizing and injection unit, into a mixing element; c) homogenizing the mixture of plastic material and filler or reinforcement materials in the mixing element, by a mixer, which is disposed movably in the mixing element and which is driven by a mechanical drive; and d) expelling the homogenized mixture of plastic material and filler or reinforcement materials from the mixing element into an injection molding tool.
The basic idea of the invention accordingly is that a known reciprocating screw plasticizing and injection unit is followed by a xe2x80x9cdynamic mixer,xe2x80x9d the purpose of which is to achieve good homogenization in applications where long-fiber additives are to be admixed to a plastic base component. An embodiment of the proposed process combination yields the advantage that, as a result of the follow-on mixer, the admixed fibers are typically never mechanically stressed to the point of being damaged or broken, either in the mixer itself or in the screw unit. The embodiment of the proposed process sequence gently transports long-fiber components which are to be admixed to the plastic, without excessive mechanical stress, andxe2x80x94in the mixerxe2x80x94homogeneously mixes these with the base mass to generate a homogeneous, or substantially homogenous product.
The screw unit here is primarily responsible for plasticizing and admixing the additive substances, while the homogenization itself takes place primarily in the follow-on xe2x80x9cdynamic mixer.xe2x80x9d
According to a first development, in accordance with the invention, it is proposed that the filler or reinforcement materials are conducted to the already at least partially melted plastic material in the preplasticizing cylinder. The fill or reinforcement materials are here added to the preplasticizing cylinder at a point where melted plastic is already present, but where nevertheless the screw channels of the plasticizing and injection screw are still effective.
It is especially advantageous if the above step d) comprises the following component steps: d1) expelling the homogenized mixture of plastic material and filler or reinforcement materials from the mixing element into a melt storage unit; d2) accumulating in the melt storage unit an amount of mixture sufficient to produce a molded part; and d3) expelling the accumulated mixture from the melt storage unit into the injection molding tool.
A mixture of plastic and additive is thus collected in the melt storage unit and is not injected into the mold until sufficient material for a shot is present.
The invention furthermore can specify that the mixture of plastic material and filler or reinforcement material, which has been expelled from the mixing element or from the melt storage unit, has admixed to it another plastic component before being injected into the injection molding tool. This is of special interest if a part is to be injection molded from several plastic components.
It is further advantageous that the mixing element is filled up by at least two parallel reciprocating screw plasticizing and injection units, of which at least one reciprocating screw plasticizing and injection unit processes a mixture of plastic material and filler or reinforcement material.
For good mixing in the xe2x80x9cdynamic mixer,xe2x80x9d it can be specified that the mixer performs a rotational motion at a time-varying rate, according to rotation per minute (rpm) profile that is stored in the control of the injection molding machine. In particular, it can be specified that the mixer does not rotate while the mixture is being plasticized.
The apparatus for injection molding parts which include a mixture of at least one plastic material and at least one additive in the form of fibrous or powdered fillers or reinforcement materials, especially glass fibers, carbon fibers, wood fibers, chalk, mineral fillers, and/or talc, comprises: at least one reciprocating screw plasticizing and injection unit for plasticizing a mixture of plastic material with filler or reinforcement materials contained therein, by means of a plasticizing and injection screw, which is disposed in a preplasticizing cylinder so that it can move rotationally and axially, and an injection molding tool, into which the plasticized mixture of plastic material and filler or reinforcement materials is injected.
The apparatus features a mixing element disposed between the reciprocating screw plasticizing and injection unit and the injection molding tool, the mixing element containing a mixer which is driven by a mechanical drive, to homogenize the mixture of plastic material with the filler or reinforcement materials contained therein.
A development specifies that an infeed device for the additive is present to feed filler or reinforcement materials into the already at least partly melted plastic material, by the infeed device conducting the filler or reinforcement materials into the region of the preplasticizing cylinder.
It is further advantageous to have a melt storage unit disposed between the mixing element and the injection molding tool. The melt storage unit has means for injecting the accumulated plastic material into the injection molding tool.
For processing at least two plastics, it is advantageous that at least one other reciprocating screw plasticizing and injection unit is present to admix another plastic component to the mixture of plastic material and filler or reinforcement materials, before their being injected into the injection molding tool.
Here, it can be advantageous that at least two parallel reciprocating screw plasticizing and injection units are present. (By parallel is meant the function and not the geometric arrangement).
As regards the design of the screw, several arrangements have been recognized as suitable: First, it can be specified that the plasticizing and injection screw is designed as a triple or multi-zone screw. Further, it is appropriate that the plasticizing and injection screw is designed as a screw equipped with a mixing section or a barrier section, or a barrier and mixing section. Finally, it is advantageous that the plasticizing and injection screw is designed as a venting screw.
Plastic mixtures are processed especially appropriately if it is specified that the infeed device is designed as a transport screw or metering device. The ejection section of the infeed device can be disposed in the venting section of the plasticizing and injection screw.
Finally, it can be specified that a check valve, especially a ball check valve, or a seal, especially a cross-pin seal, is disposed between the mixing element and the melt storage unit.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.